This invention relates generally to a friction transmission and more particularly to a friction transmission with hydraulic generation of the pressing forces between friction members.
Such friction transmissions serve primarily for effecting a stepless, or infinite adjustment of the transmission ratio. To give only a few examples here, reference may be made to cone-pulley belt-type transmissions with traction means circulating between cone-pulley pairs, cone-pulley transmissions with friction discs parallel with one another, and having rotation axes which are offset in relation to one another and between which balls guided in a cage are displaceable for transmission ratio adjustment, or friction disc transmissions with friction discs rolling one upon the other, the axes of rotation of which are arranged at right angles to one another.
It is common to all friction transmissions that the force with which the friction members are pressed against one another is to be adjusted according to load, in order to obtain as exactly as possible the necessary pressing force, since excess pressure leads to an unnecessary wear of the friction members and their mountings, while inadequate pressing causes mutual slipping and thus again excessive wear of the friction members.
Now in the friction transmissions known hitherto the new problem has arisen that on the occurrence of torque surges, an unduly long time delay occurs in the build-up of the requisite hydraulic pressure, and thus the pressing force, between the friction members. Thus slipping of the friction partners in relation to one another, with corresponding damage, becomes unavoidable.
The cause of the delay in pressure build-up in the case of a torque surge resides substantially in the elastic yieldability of the components, and more specifically is attributable to the facts that: the traction means elongates; the axial supporting yields; the oil leakage quantity increases; presser cylinder and conduits expand; and the oil is somewhat compressible. The extra demand for pressure medium, necessary thus at the increased pressure level caused by a torque surge, amounts in a medium-power transmission to several cubic centimeters.
In order to make this quantity of oil available in the requisite time of a few milliseconds, a pressure-medium pump would be necessary having an output increased substantially in comparison with the normal requirement. Another possibility would consist in making the elastic components so rigid that the requisite extra demand for oil is reduced to an acceptable amount. Both measures, and also their combination, are unacceptable however for economic reasons.